Color, digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). In a common OLED color display device a pixel includes red, green, and blue colored OLEDs. By combining the illumination from each of these three OLEDs in an additive color system, a full-color display having a wide variety of colors can be achieved.
OLEDs may be used to generate color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum. However, the known red and blue emissive materials are not particularly power efficient. In fact, broad bandwidth (white appearing) materials are known that have power efficiencies that are high enough by comparison to narrow bandwidth materials to produce a comparably power efficient OLED display by placing color filters over a broad bandwidth emissive material. Therefore, it is known in the art to produce OLED displays by building a display using an array of white-emitting OLEDs and placing color filters over the OLEDs to achieve red, green and blue light emitting elements in each pixel. It is also known in the art to effectively create color filters using optical effects, such as microcavities, to amplify light emission within the spectrum.
White-light emitting OLED devices that are known in the art are typically formed by doping multiple emitting layers such that each doped layer produces light within a specific spectral frequency band. White-light emitting devices formed from three individual emitting layers, e.g., are known in the art and can be produced by doping individual layers to produce spectral peaks in the red, green and blue portions of the spectrum. These devices have the advantage that the spectral emission of the white-light emitting devices can be matched to the peaks in the spectral transmission function of the red, green, and blue color filters typically used with white-light emitting devices, which optimizes the efficiency of the overall device. However, these devices require significantly more manufacturing steps than would be required if a white material can be formed by doping only two emissive layers as each doped emission layer is typically formed by applying multiple coating steps, including a host, dopant, an optional co-dopant and an optional connecting layer.
It is also known that three-color devices can be produced by creating a white-light emitting OLED material from only two doped layers. This reduction in the number of doped light emission layers significantly reduces the manufacturing complexity of the OLED display device. In white-light emitting OLED structures that are formed from two doped layers, one of these layers typically produces one or more broad peaks that emit cyan or yellow light. In such devices, the yellow peak provides energy to both the green and red light emitting elements and the cyan peak provides energy to the blue and green light emitting elements. Unfortunately, in these devices, the peak in the spectral energy distribution for the cyan or yellow light emitting layers can not be matched to the peak in the spectral transmission function for the color filters. For this reason, much of the energy emitted by the device is absorbed by the color filters rather than being transmitted to the observer, resulting in a display device with lower energy efficiency, particularly when narrow-band color filters are employed in order to create a display device with good color saturation and a large color gamut volume.
In a typical, prior-art OLED display, it is known that the luminance of the red, green, and blue OLEDs increase as current density delivered to the OLED is increased. Therefore, to increase the luminance of the display, one must increase the current delivered to an OLED with a given area. Unfortunately, increasing the current density used to drive an OLED not only increases the power required to drive the OLED but also reduces the lifetime of the OLED since the lifetime of an OLED emitter, as is known in the prior art, depends on the total current driven through the OLED emitter. Therefore, increasing the luminance of an OLED display not only increases the power needed to drive the OLED display device but can significantly reduce the lifetime of the OLED display device.
Traditionally, display devices have been constructed from a triad of red, green, and blue light emitting elements that, together, form a pixel and define a color gamut. The peak wavelengths of these light emitting elements will typically be in the short wavelength portion of the visible spectrum for blue, the middle wavelength portion of the visible spectrum for green, and the long wavelength portion of the visible spectrum for red. Given that the relative radiant efficiency of these light emitting elements are typically similar and the fact that the eye is most sensitive to energy in the middle wavelength portion of the visible spectrum, the green light emitting element will typically have significantly higher luminance efficiency than the red or blue light emitting elements.
While one goal when designing an OLED display device is to minimize the power consumption by maximizing the efficiency of each OLED, a competing goal is to maximize the color gamut of a display device. To improve the color gamut of the display device, the peak wavelength of the blue light emitting element will typically be reduced, providing energy that is even shorter in wavelength and further reducing the eye's sensitivity to the radiant energy provided by the light emitting element. Similarly, to increase the color gamut, the peak wavelength of the red light emitting element must be increased, producing energy that is even longer in wavelength and further reducing the eye's sensitivity to the radiant energy provided by the light emitting element. For this reason, the goals of providing increased color gamut and reduced power consumption typically compete with one another.
Another important factor when designing a display device is that many of the colors that must be produced will be neutral or desaturated. That is, these colors will be at or near the white point of the display. For example, it is known that the predominant color on many graphic displays is white. This includes the backgrounds in many popular applications and operating systems. Additionally, pictorial images tend to be composed of neutral or desaturated colors.
Therefore, to decrease the power consumption of a display device under typical use conditions, it is important that colors near the white point of the display device consume as little power as possible. However, in a typical three-color display device, white and desaturated colors are produced by the addition of luminance from the red, green, and blue light emitting elements. Since the red and blue light emitting elements typically have relatively low luminance efficiency, as discussed earlier, the power consumption of the display device will be near its maximum when displaying white or a desaturated color.
However, it is possible to utilize an additional light emitting element with a higher luminance efficiency than at least one of the light emitting elements. U.S. Pat. No. 6,693,611 by Burroughes, Feb. 17, 2004 as well as U.S. Patent Application 2003/0011613 by Booth, Jan. 16, 2003 describe a display device with red, green, blue and cyan light emitting elements. These disclosures discuss the fact that blue light emitting elements typically have a lower luminance efficiency than a cyan emitter due to the fact that the eye is more sensitive to light that is emitted in the spectral frequencies that must be used to create cyan light than to light that is emitted in the spectral frequencies that are used to create blue light. However, neither of these references discuss OLED display devices that are constructed from white materials with color filters to create light emission for either color, nor do they discuss any reason why the radiant efficiency of the materials used to create a cyan light emitting element may be higher than the radiant efficiency of the materials used to create a blue light emitting element.
OLED display devices having other than red, green, and blue light emitting elements have also been discussed by others. For example, U.S. Pat. No. 6,570,584 by Cok, et al., May 27, 2003 describes OLED display devices having an additional cyan, yellow, and or magenta OLEDs that are utilized to increase the color gamut of the display device. Cok et al. does not discuss the formation of this display device from a white emitting material that is filtered by color filters to produce an OLED display device.
U.S. Patent Application 2002/0191130 by Liang et al, Dec. 19, 2002 discusses an OLED display device employing pairs of complementary colors. Once again, this patent application does not discuss the formation of this display device from a white-emitting material that is filtered by color filters to produces an OLED display device.
There is a need, therefore, for an improved color OLED display device enabling improved power efficiency and/or lifetime that is formed from a white-light emitting layer and a means for selectively emitting light.